Fitz--Muscle Receptors Flashcards
Group Ia afferents
- (also called primary afferents)
- innervate all 3 types of intrafusal fibers
- provide information about both length and velocity.
golgi tendon organs
- a specialized receptor that is located between the muscle and the tendon
- signals information about the load or force being applied to the muscle
stretch reflex (myotatic reflex)
muscle stretch –> activation of muscle spindle Ia afferent –> enter spinal chord via dorsal root ganglion (sensory) –> synapse on alpha-MN in ventral horn (motor) –> muscle contraction
monosynaptic activation
Group II afferents
- (also called secondary afferents)
- innervate the ends of the nuclear chain fibers and the static nuclear bag fibers at specialized junctions termed flower spray endings.
- Do not innervate the dynamic nuclear bag fibers,
- signal information about muscle length only.
gamma motor neuron
** innervate intrafusal fibers**, which contract only slightly
gamma activation of the intrafusal fiber is necessary to keep the muscle spindle taut, and therefore sensitive to stretch
what two muscle components work together to maintain tone?
spindle afferents and golgi tendon organs
flaccid paralysis
elimination of alpha motoneurons
spastic paralysis
overactive gamma motoneurons
reflex arc mechanism
- sensory receptor
- primary afferent neuron
- 1-3 synapses
- motor neuron
- muscles
withdrawal reflex
noxious stimulus –> withdrawal of limb –> contraction of flexors and relaxation of extensors
In spinal chord lesions reflexes…
are functional below lesion
initially –> spinal shock,
eventually –> reflexes return (simplist first)
Alpha motor neurons
- (also called lower motor neurons) innervate extrafusal fibers (skeletal muscle) and cause the muscle contractions that generate movement.
- release acetylcholine at neuromuscular junction
- anterior horn of the spinal cord and motor nuclei of the brainstem
- only neuron through which the motor system can communicate with the muscles
How does a motor neuron control the amount of force that is exerted by muscle fibers?
rate code
increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force
Muscle Spindles
specialized receptors that signal
(a) the length and
(b) the rate of change of length (velocity) of the muscle
intrafusal fibers
are muscle spindles
extrafusal fibers are the large majority of muscle fibers that allow the muscle to do work
There are 3 types of muscle spindle fibers:
Nuclear Chain fibers
Static Nuclear Bag fibers
Dynamic Nuclear Bag fibers
Nuclear Chain fibers. Nuclei are aligned in a single row (chain) in the center of the fiber. Signal information about the static length of the muscle.
Static Nuclear Bag fibers. Nuclei are collected in a bundle in the middle of the fiber. Like the nuclear chain fiber, these fibers signal information about the static length of a muscle.
Dynamic Nuclear Bag fibers. Anatomically similar to the static nuclear bag fibers, but they signal primarily information about the rate of change (velocity) of muscle length.
inverse stretch reflex, where strong contraction is followed by muscle relaxation…
- due to GTO afferents (1b) activation of an inhibitory interneuron
- stretch –> depolarization –> increased inhibitory interneuron –> decrease alpha motor neuron activity –> relaxation
In summary,
muscle spindle vs. golgi tendon organ
- Muscle spindles signal information about the length and velocity of a muscle
- Golgi tendon organs signal information about the load or force applied to a muscle
Reciprocal inhibition in stretch reflex
mechanism through which the opposing muscle group is inhibited
accomplished by an inhibitory interneuron in the spinal cord
Ia afferent of the muscle spindle bifurcates –> Ia inhibitory interneuron –> inhibits alpha-MN to opposing muscle (relaxation)
Golgi tendon organ is involved in a spinal reflex known as the autogenic inhibition reflex. When tension is applied to a muscle, how does this work?
Afferent Ib fibers activated –> sensory (DRG) to dorsal column –> synapses onto Ib inhibitory interneuron –> inhibits alpha motor neuron that innervates the same muscle that caused the GTO Ib afferent to fire.
Ib fibers (sensory) also bifurcate, and excite an interneuron to alpha-MNs of the opposing muscle.
Remember that GTOs respond to force!
Flexor reflex (withdrawal reflex)
Pain –> Group III afferents (sensory/DRG) –> excitatory interneuron –> activation of alpha-MNs to flexor muscles (up and down spinal cord)
Note that Group III afferents also activate inhibitory interneurons to extensor alpha-MNs.
